XDSL system having selectable hybrid circuitry

ABSTRACT

An XDSL system comprising a hybrid circuit in operative communication with a transmission line and an XDSL modem associated with a subscriber premises. The hybrid circuit comprises a plurality of selectable impedance circuits having different impedance values. A controller selectively engages each of the impedance circuits in-line with the XDSL modem and the transmission line. The controller records the performance of the modem with each circuit engaged, and selects the desired impedance circuit as a function of the performance characteristics associated with each of the impedance circuits. The impedance circuit which provides the best performance characteristic is selected as the impedance circuit for normal modem operation.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/177,925 entitled “XDSL System Having SelectableHybrid Circuitry” filed Jan. 25, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to data transmission systems, andmore particularly, concerns selectable hybrid circuitry for XDSLtransmission systems.

BACKGROUND OF THE INVENTION

[0003] Digital Subscriber Line (DSL) is a technology which allows forsimultaneous voice and data traffic to coexist over a communicationchannel comprising a standard telephone transmission line. Typically,the standard telephone transmission line comprises an unshieldedtwisted-pair of copper wire having a gage of 22-26 AWG. Twisted pairs,which can be used to connect a central telephone system to asubscriber's telephone system can support bandwidths up to 2 MHz throughthe use of digital signal processing (DSP) technology. Thus, they can beused for bandwidth-intensive applications, such as Internet access andvideo-on demand, as well as for carrying voice traffic. Frequencydivision multiplexing is used so that the plurality of signals, eachoccupying a different frequency band, can be simultaneously sent overthe same transmission line.

[0004] Because there are different varieties of digital subscriber linetechnology, it is sometimes generally referred to as XDSL wherein the“X” refers to a specific DSL standard such as HDSL for high bit ratedigital subscriber line or RADSL for rate adaptive digital subscriberline, etc. As the name implies, ADSL is asymmetric in that the datatransmission rates differ in the upstream and downstream direction. Inthe context of a phone system, the downstream direction is defined astransmissions from the central office to a remote location that may bean end user such as a residence or business. The reverse signalcorresponds to transmissions in an upstream direction, i.e., from theremote location to the central office.

[0005] ADSL data traffic bandwidth for CAP (carrier-less amplitude andphase) modulation is typically from about 35 kHz to 1.5 MHz. Thebandwidth for ADSL data traffic using DMT (discreet multi-tonemodulation) is from approximately 25 kHz to 1.5 MHz. A hybrid circuit istypically used in DSL systems to separate the upstream (transmit) anddownstream (receive) signals no the twisted-pair transmission line.

[0006] Referring now to FIG. 1 there is shown an example of a typicalanalog front end of an ADSL remote unit (ATU-R). The hybrid circuitry 2separates the signals received on the two-wire transmission line 4 intorespective upstream and downstream communication channels. The upstreamcommunication channel occupies a bandwidth from about 30 kHz to 138 kHzas represented by the band pass filter 6. The downstream communicationchannel occupies a bandwidth from approximately 170 kHz and upward asrepresented by the high pass filter 8. In such cases, the upstream anddownstream frequency arranges are only separated by 32 kHz. Accordingly,it is very important that the impedence of the hybrid 2 matches theimpedence of the transmission line 4 to prevent reflections which cansignificantly degrade performance, particularly in the downstreamdirection. This can occur when high upstream signal power in the 138 kHzrange is reflected back through a poor hybrid match of the transmissionline into the downstream path thereby interfering with downstream signalrecovery. Current ADSL modem implementations use only one impedencevalue for the hybrid circuit 2 which is specifically tuned to acharacteristic line impedence of 100 ohms.

[0007] As a practical matter, however, the impedence of the transmissionline 4 is rarely 100 ohms due to line conditions such as 10 bridgedtaps. A bridged tap is a non-terminated copper pair cable connected inparallel to the subscriber line. While a bridged tap has no effect onPOTS service, it can significantly alter the impedence of thetransmission line thereby creating an impedence mismatch with existingADSL modem hybrids.

[0008] Thus, there exists a need for an XDSL system having selectablehybrid circuitry which closely matches the particular subscribertransmission line impedence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a more complete understanding of this invention, referenceshould now be made to the embodiments illustrated in greater detail inthe accompanying drawings and described below by way of examples of theinvention.

[0010] In the drawings:

[0011]FIG. 1 is a block diagram of an XDSL system incorporating a hybridcircuitry according to the prior art.

[0012]FIG. 2 is a block diagram of an XDSL system incorporatingselectable hybrid circuitry according to one embodiment of the presentinvention.

[0013]FIG. 3 is a logic flow diagram of one method of controlling theselectable hybrid circuitry of FIG. 2.

[0014]FIG. 4 is a block diagram of an XDSL system incorporatingselectable hybrid circuitry according to another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Broadly speaking, the invention includes selectable hybridcircuitry for an XDSL modem system wherein each hybrid circuit has adifferent termination impedence. Upon initialization, the XDSL modemsynchronizes with the different hybrid circuits and records the upstreamand downstream performance for each hybrid. After the initializationsequence is complete, the modem performs a final synchronizationcorresponding with the best performing hybrid circuit.

[0016] Referring now to FIG. 2, there is shown a block diagram of anXDSL system incorporating selectable hybrid circuitry according to oneembodiment of the present invention. The diagram of FIG. 2 is dividedinto two portions; one of which represents the telco central office 10and the other which represents the subscriber premises 12. Communicationchannel 14 represents the main feeder cables to the telco outside plant.

[0017] These cables interface with the main distribution frame (MDF) 16within a central office 10. A splitter 18 divides the plain oldtelephone service (POTS) signals from the higher frequency XDSL signals.The POTS signals are routed through the central office switch to theworldwide public switched telephone network 20. The XDSL signals arerouted to a plurality of XDSL transceiver units at the central office(XTU-C) 22 each of which corresponds to a respective XDSL transceiverunit remotely located (ATU5 R) at a subscriber premises. The XTU-Cs 22connect through the central office XDSL subscriber loop and data networkinterface to access the Internet ISP and other data networks 24.

[0018] At least a portion of the communication channel that connects thecentral office 10 with the subscribed premises 12 comprises atwisted-pair subscriber loop 26. The frequency bands transmitted alongthe twisted-pair subscriber loop 26 between a central office 10 and asubscriber premises 12 include the low frequency POTS band, the higherfrequency upstream data communications band and the higher frequencydownstream data communication band. The twisted-pair subscriber loop 26connects to the subscriber premises through a standard RJ 11 modularconnector 28. A splitter 30 comprising a high pass filter 31 and a lowpass filter 32 may also be included at the subscriber premises to dividethe higher frequency data communication bands from the lower frequencyPOTS communication band. The POTS signals are then transmitted to POTSdevices 34 such as a telephone or facsimile machine. The higherfrequency data signals are communicated to the XTU-R 36. Of course, in asplitter-less XDSL configuration, the splitter 30 would not be necessaryand the XTU-R 36 would incorporate a device for filtering out the POTSband communication signals.

[0019] The XTU-R 36 comprises the selectable hybrid circuitry 40 whichincludes preferably four hybrid circuits H1 H2 H3 H4. In accordance withthe control scheme described below, the hybrid circuit having theclosest matching impedence to the two wire transmission line 38 isselected to be included in the transmission path. The output of thehybrid circuitry 40 separates the transmit and receive signals into anupstream data path 42 and downstream data path 44. The selection controlof the hybrid circuitry 40 preferably implemented as firmware executedby the XTU-R controller which comprises a microprocessor or a digitalsignal processor 46.

[0020] Referring now to FIG. 3, there is shown a logic flow diagram ofone method of controlling the selectable hybrid circuitry 40 of FIG. 2.The logic begins in step 80 when the XTU-R modem is power cycled. Uponpower-up, the logic enters a training routine to select the hybridcircuit characteristics which provide the best transmission performance.Thus, in step 82, the first hybrid circuit H1 is selected. In step 84,the performance characteristics of the communication channel isdetermined utilizing the first hybrid circuit. The performancecharacteristics selected for analysis can be any known performancecharacteristic such as the downstream data rate, upstream data rate, theline attenuation, and the noise margin. The desired performancecharacteristics are then saved for later comparison with the performancecharacteristics of the remaining hybrid circuit configurations.

[0021] In step 86, the next hybrid circuit is selected such as, forexample, H2 of FIG. 2. The line performance characteristics are thendetermined using hybrid circuit H2. This continues until all of thehybrid Circuit configurations within the selectable hybrid 40 areexhausted.

[0022] Alternatively, if the determined performance characteristics of,for example, the first hybrid circuit, exceed a predetermined thresholdline characteristic, no further hybrid circuit configurations need betested. The logic then continues to step 88 wherein the hybrid circuitconfiguration having the best performance as determined by the selectedperformance criteria is selected as the configuration for all subsequenttransmissions. Again, alternatively, if during the testing phase, ahybrid circuit configuration exceeds a predetermined line performance,that hybrid circuit configuration under consideration can be selected asthe hybrid circuit configuration for all subsequent transmissions.

[0023] Preferably, the performance and selection analysis is performedonly when the modem is power cycled. In this way, any disturbances thatcause a retrain such as a line disconnect, will not result in a hybridselection change.

[0024] Referring now to FIG. 4, there is shown a selectable hybridcircuit according to another embodiment of the present invention. Theconfiguration shown in FIG. 4 is an analog front end topology whichsupports an Alcatel chip set, analog front end for an ADSL receiverunit. For such a configuration, Z2 represents an impedence equal to 787ohms+100 nF. ZIN represents the input impedence of the downstreamreceive path high pass filter. Drivers 100 are receive path downstreamdrivers. ZLINE represents the input impedence of the two wiretransmission line. Drivers 102 represent the transmit path upstreamdrivers.

[0025] The hybrid circuitry comprises an analog switch 104 andcorresponding plurality of impedence values ZH1, ZH2, ZH3, ZH4.Preferably, the switch 104 is an analog switch such as model numberADG409 available from Analog Devices, Inc. The ADG409 switch has aresistance of approximately 40 ohms, and this must be taken into accountwhen selecting the corresponding hybrid impedances ZH1 through ZH4.ASIC1 and ASIC2 are binary select lines for the switch 104. These binaryselect lines are controlled by the processor which cycles through abinary count on successive hybrid circuit selections and performancedeterminations to select the best hybrid circuit configuration.

[0026] Preferably, the values for ZH1 through ZH4 are selected toprovide the closest impedence match to the most common impedencecharacteristics of subscriber loops. As described above, bridged tapsare found commonly within the public switch telephone network. Accordingto studies, approximately 75% of all subscriber loops have some type ofbridged tap, and approximately 33% of all loops have bridged tapsbetween 250 and 500 feet from the subscriber premises. The existence ofbridged taps can significantly alter the impedence characteristics ofthe subscriber loop. For example, a 15 kFT, 26 AWG cable without abridged tap exhibits an input impedence of 118 ohms at 200 kHz. With a350 ft bridged tap, however, the impedence changes to 71 ohms. Thisrepresents a nearly 40% impedence change. With the assumptions shown inFIG. 4 for the Alcatel chip set and the ADG 409 switch, the preferredvalues for ZH1 through ZH4 are as follows:

ZH1=750Ω//(525Ω+2000 pF).

[0027] This value for ZH1 best matches an 8 to 20 kft 24/26 AWG cablewith a 350 ft bridged tap.

[0028] Preferably, the value of ZH2 is 620Ω//(320Ω+4200 pF). This valueof ZH2 best matches the impedence characteristics of a 12-15 kft, 26 AWGcable with a 350 bridged gap.

[0029] Preferably, the value of ZH3 is 460Ω//(1200Ω+520 pF). This valueof ZH3 best matches a 24/26 AWG cable without bridged taps.

[0030] Finally, the preferred value of ZH4 is 620Ω//(680Ω+2200 pF). Thisvalue for ZH4 represents a compromise impedence value which combinescharacteristics of the other three impedence values.

[0031] Of course, the number of hybrid circuit configurations shown andthe value of each corresponding impedence is merely exemplary. It shouldbe understood that more or less than four hybrid circuit configurationsare possible and that each circuit configuration can have a differentimpedence value. The more circuit configurations implemented for aselectable hybrid design, however, the slower the analysis upon power-upsince the performance of each hybrid configuration is tested to selectthe best performing hybrid configuration.

[0032] From the foregoing, it can seen that there has been brought tothe art a new and improved selectable hybrid circuit for use in an XDSLsystem which provides improved performance by more closely matching theimpedence characteristics of the twisted-pair subscriber loop. While theinvention has been described in connection with one or more embodiments,it should be understood that the invention is not limited to thoseembodiments. On the contrary, the invention covers all alternatives,modifications, and equivalence, as may be included within the spirit andscope of the appended claims.

What is claimed is:
 1. An XDSL system comprising: a hybrid circuit inoperative communication with a transmission line and an XDSL modemassociated with a subscriber premises, said hybrid circuit comprising aplurality of selectable impedance circuits; and a switch for connectingeach of said plurality of selectable impedance circuits in-line withsaid XDSL modem and said transmission line in response to a controlsignal.
 2. The XDSL system of claim 1 further comprising a controllerfor producing said control signal as a function of a performancecharacteristic associated with each of said impedance circuits.
 3. TheXDSL system of claim 1 wherein the plurality of selectable impedancecircuits equals four.
 4. The XDSL system of claim 1 wherein one of saidplurality of impedance circuits has an impedance value equal to acharacteristic line impedance of said transmission line without abridged tap.
 5. The XDSL system of claim 4 wherein one of said pluralityof impedance circuits has an impedance value equal to a characteristicline impedance with a bridged tap.
 6. The XDSL system of claim 2 whereinsaid performance characteristic is a data transmission rate and saidcontrol signal corresponds to the respective impedance circuitassociated with the highest data transmission rate value.
 7. A method ofconfiguring an XDSL system comprising: providing a hybrid circuitin-line with a transmission line and an XDSL modem associated with asubscriber premises, said hybrid circuit comprising a plurality ofselectable impedance circuits; and engaging one of said plurality ofselectable impedance circuits in-line with said transmission line andsaid XDSL modem in response to a control signal.
 8. The method of claim7 wherein the step of engaging includes the step of engaging seriatimlyeach of said plurality of impedance circuits in-line with saidtransmission line and said XDSL modem.
 9. The method of claim 8 furthercomprising the step of determining a performance characteristic of saidXDSL system for each of said plurality of impedance circuits whenengaged, and outputting said control signal as a function of each ofsaid performance characteristics.
 10. The method of claim 9 wherein saidperformance characteristic is a data transmission rate and said controlsignal corresponds to the respective impedance circuit associated withthe highest data rate.
 11. The method of claim 9 wherein saidperformance characteristic is a data transmission rate and said controlsignal corresponds to a first respective impedance circuit associatedwith a data rate greater than a selected rate.
 12. The method of claim 9wherein the step of outputting includes the step of comparing each ofsaid performance characteristics associated with each respectiveimpedance circuit.
 13. The method of claim 7 wherein one of saidplurality of impedance values is equal to a characteristic lineimpedance of said transmission line without a bridged tap.
 14. Themethod of claim 13 wherein one of said plurality of impedance values isequal to a characteristic line impedance with a bridged tap.
 15. An XDSLsystem comprising: a hybrid circuit in operative communication with atransmission line and an XDSL modem associated with a subscriberpremises, said hybrid circuit comprising a plurality of selectableimpedance circuits; a switch for connecting each of said plurality ofselectable impedance circuits in-line with said XDSL modem and saidtransmission line in response to a control signal; and a controllerprogrammed to determine a performance characteristic associated witheach of said plurality of selectable impedance circuits when connected,and output said control signal as a function of said performancecharacteristics associated with each of said impedance circuits.
 16. TheXDSL system of claim 15 wherein the number of impedance circuits isfour.
 17. The XDSL system of claim 15 wherein said performancecharacteristic is a transmission data rate.
 18. The XDSL system of claim15 wherein said performance characteristic is a transmission lineattenuation.
 19. The XDSL system of claim 15 wherein said performancecharacteristic is a noise margin.
 20. The XDSL system of claim 15wherein one of the plurality of impedance circuits comprises a 460 ohmresistor in parallel with a 1200 ohm resistor and 520 pF capacitor.